1. Field of the Invention
The present invention relates to an optical transmission line through which a plurality of wavelengths of optical signals propagate in a transmission system utilizing a wavelength division multiplexing (WDM) technique.
2. Related Background Art
WDM transmission systems enable high-speed, large-capacity optical communications by transmitting a plurality of wavelengths of optical signals (WDM signals). Silica type optical fibers utilized as optical transmission lines in such WDM transmission systems have been known to lower their transmission loss near a wavelength of 1.55 xcexcm (1550 nm). Also, since optical amplifiers amplifying optical signals in a wavelength band of 1.55 xcexcm have been in actual use, optical signals in the 1.55-xcexcm wavelength band are used in general.
In optical transmission lines, if chromatic dispersion occurs in a wavelength band in use (e.g., 1.55-xcexcm wavelength band), then signal waveforms are deformed, whereby transmission characteristics may deteriorate. Therefore, from the view point of preventing signal waveforms from being deformed, it is desirable that the absolute value of chromatic dispersion in the wavelength band in use be smaller. If the absolute value of chromatic dispersion in the wavelength band in use is not greater than a predetermined value, on the other hand, then four-wave mixing, which is a kind of nonlinear optical phenomena, is likely to occur. The occurrence of four-wave mixing generates cross talk and noise, thereby eventually deteriorating transmission characteristics. For suppressing the occurrence of four-wave mixing, the repeater intervals for arranging optical amplifiers may be shortened, and the optical power of optical signals emitted from the optical amplifiers may be lowered. However, for realizing this, it is necessary to install a large number of optical amplifiers, which lowers the cost performance of the whole system.
For dealing with problems such as those mentioned above, U.S. Pat. No. 5,894,537 discloses an optical transmission line in which respective sections having positive and negative chromatic dispersions at a predetermined wavelength in the wavelength band in use are alternately disposed along its longitudinal direction. When such an optical transmission line is used, transmission characteristics can be restrained from deteriorating due to chromatic dispersion if the average chromatic dispersion observed as the whole system is set such that its absolute value does not exceed a predetermined value. It has also been presumed that the deterioration of transmission characteristics caused by nonlinear optical phenomena such as four-wave mixing can be suppressed if the absolute value of chromatic dispersion is set to a predetermined value or higher in most of the sections of the optical transmission line.
Also, a referencexe2x80x94Y. Kubo, et al., xe2x80x9cDispersion Flattened Single-Mode Fiber for 10,000 km Transmission System,xe2x80x9dECOCxe2x80x290 (1990)xe2x80x94which will herein after be referred to as Kubo reference, describes an optical transmission line including respective sections in which both chromatic dispersion and dispersion slope at a predetermined wavelength in the wavelength band in use are positive and negative. When such an optical transmission line is employed, the average chromatic dispersion observed as the whole system can be set such that its absolute value does not exceed a predetermined value in a wide wavelength band. As a result, the deterioration in transmission characteristics caused by chromatic dispersion is suppressed over this wide wavelength band.
The inventors have studied the above-mentioned conventional techniques and, as a result, have found problems as follows. The optical transmission line disclosed in the above-mentioned U.S. Pat. No. 5,894,537 can set the average chromatic dispersion observed as the whole system, such that its absolute value does not exceed a predetermined value at a predetermined wavelength in a wavelength band in use. At other wavelengths in the wavelength band in use, which are different from the above-mentioned predetermined wavelength, however, the absolute value of average chromatic dispersion observed as the whole system becomes greater, thereby yielding a possibility that the deterioration of transmission characteristics caused by chromatic dispersion cannot effectively be suppressed over the whole wavelength band in use. In particular, as the difference between the above-mentioned predetermined wavelength and other wavelengths included in the wavelength band in use is greater, the absolute value of average chromatic dispersion observed as the whole system becomes greater. Therefore, even when the optical transmission line disclosed in the above-mentioned U.S. Pat. No. 5,894,537 is utilized, there is a possibility that a WDM transmission system having a wide wavelength band in use is hard to realize.
The optical transmission line disclosed in the above-mentioned Kubo reference can set the average chromatic dispersion observed as the whole system, such that its absolute value does not exceed a predetermined value in a wide wavelength band. When the optical transmission line is seen section by section, however, there may exist a point where the absolute value of chromatic dispersion falls short of the predetermined value, thus leaving a possibility that transmission characteristics may locally deteriorate due to nonlinear optical phenomena such as four-wave mixing. Also, depending on the sequence of connection of respective optical fibers whose chromatic dispersion and dispersion slope are both positive and negative (the positional relationship of thus arranged two kinds of optical fibers observed in the traveling direction of propagating optical signals), accumulative chromatic dispersion may increase locally, thus yielding a possibility of transmission characteristics deteriorating due to an interaction between the accumulative chromatic dispersion and nonlinear optical phenomena.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical transmission line comprising a structure which can yield favorable transmission characteristics over a wider wavelength band in use.
The optical transmission line according to the present invention comprises one or more first waveguides and one or more second waveguides having optical characteristics opposite to each other at a predetermined wavelength in a wavelength band in use, and also comprises a structure in which the first and second waveguides are alternately arranged along a traveling direction of optical signals in the wavelength band in use. Namely, the optical transmission line includes a section in which the first and second waveguides are arranged adjacent each other along the traveling direction of the optical signals in the wavelength band in use. In particular, each first waveguide has a chromatic dispersion with a sign opposite to that of the chromatic dispersion of each second waveguide and a dispersion slope with a sign opposite to that of the dispersion slope of each second waveguide.
The optical transmission line according to the present invention is disposed in at least one of places between an optical transmitter for emitting a plurality of wavelengths of optical signals and a receiver for receiving the optical signals, between the optical transmitter and a repeater station including an optical amplifier or the like, between repeater stations, and between the repeater station and the receiver. Also, the optical transmission line can be constituted by a unitary optical fiber having no junction, as well as a configuration in which a plurality of optical fibers functioning as each of the first waveguides and a plurality of optical fibers functioning as each of the second waveguides are fusion-spliced in a predetermined sequence. Along the traveling direction of optical signals, the unitary optical fiber as the optical transmission line is alternately formed with one or more first parts respectively corresponding to the f first waveguides and one or more second parts respectively corresponding to the second waveguides. Namely, in the unitary optical fiber, the first and second parts corresponding to the first and second waveguides, respectively, are formed so as to be adjacent each other along the traveling direction of optical signals in the wavelength band in use.
Since the optical transmission line according to the present invention is constituted by first and second waveguides (corresponding to the first and second parts in the unitary optical fiber, respectively) having respective chromatic dispersions with signs opposite to each other and respective dispersion slopes with signs opposite to each other at a predetermined wavelength (e.g., wavelength of 1.55 xcexcm=1550 nm) in a wavelength band in use (e.g., 1.55[-] xcexcm wavelength band), it can be designed not only so as to lower the absolute value of average chromatic dispersion observed as the whole optical transmission line, but also so as to lower the absolute value of average chromatic dispersion observed as the whole optical transmission line. As a consequence, transmission characteristics can effectively be restrained from deteriorating due to chromatic dispersion in WDM transmission utilizing a wider wavelength band. Also, the optical transmission line includes a section in which the first and second waveguides are arranged adjacent each other along the traveling direction of the optical signal in the wavelength band in use. Such a configuration prevents chromatic dispersion with a specific sign from occurring over a long haul, and also can effectively restrain transmission characteristics from deteriorating due to the interaction between accumulative chromatic dispersion and nonlinear optical phenomena.
In particular, in the basic configuration mentioned above, it is preferred that each of the first waveguides has a chromatic dispersion with an absolute value of 1 ps/nm/km or more but 10 ps/nm/km or less at the predetermined wavelength, and that each of the second waveguides has a chromatic dispersion with an absolute value of 1 ps/nm/km or more but 10 ps/nm/km or less at the predetermined wavelength (first characteristic feature). Here, the respective chromatic dispersions of the first and second waveguides have signs opposite to each other. If chromatic dispersion is generated to a certain extent over the whole optical transmission line, then the deterioration in transmission characteristics and fluctuation in transmission loss can be restrained from occurring due to nonlinear optical phenomena.
Preferably, in the above-mentioned basic configuration, both each first waveguide and each second waveguide have a length of 0.5 km or more but 10 km or less, whereas the product of the absolute value of chromatic dispersion at a predetermined wavelength in the wavelength band in use and the length in each of the first and second waveguides is 10 ps/nm or less (second characteristic feature). In this case, the accumulative chromatic dispersion does not become a high value, whereby transmission characteristics can further be restrained from deteriorating due to the interaction between accumulative chromatic dispersion and nonlinear optical phenomena. When combined with the above-mentioned first characteristic feature, the second characteristic feature can further suppress the deterioration of transmission characteristics in the optical transmission line.
Further, in the above-mentioned basic configuration, it is preferred that both each first waveguide and each second waveguide have an effective area of 40 xcexcm2 or more at the above-mentioned predetermined wavelength (third characteristic feature). Setting the effective are a larger to a certain extent can effectively suppress the occurrence of nonlinear optical phenomena. Here, as disclosed in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area Aeff is given by the following expression:       A    eff    =      2    ⁢                            π          ⁡                      (                                          ∫                0                ∞                            ⁢                                                E                  2                                ⁢                r                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  r                                                      )                          2            /              (                              ∫            0            ∞                    ⁢                                    E              4                        ⁢            r            ⁢                          xe2x80x83                        ⁢                          ⅆ              r                                      )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the core center. The third characteristic feature can also be combined with at least one of the above-mentioned first and second characteristic features, and their combination can yield desirable effects.
In addition, it is preferred that the chromatic dispersion and dispersion slope in each of the first waveguides at the above-mentioned predetermined wavelength have an identical sign (e.g.,both are positive). Similarly, it is preferred that the chromatic dispersion and dispersion slope in each of the second waveguides at the above-mentioned predetermined wavelength have an identical sign (e.g., both are negative). If the chromatic dispersion and dispersion slope in one section (waveguide) are set so as to have the same sign, then the respective refractive index profiles of the first and second waveguides can have forms similar to each other, whereby the optical transmission line can be realized by a unitary optical fiber in which the first and second waveguides (corresponding to the first and second parts) are formed alternately. Also, the respective absolute values of average chromatic dispersion and average dispersion slope observed as the whole optical transmission line can easily be set to their predetermined values or lower.
Preferably, the average chromatic dispersion observed as the whole optical transmission line at the above-mentioned predetermined wavelength is designed so as to yield an absolute value of 3 ps/nm/km or less. This is because of the fact that it can effectively restrain transmission characteristics from deteriorating due to accumulative chromatic dispersion. Preferably, at the above-mentioned predetermined wavelength, the average dispersion slope observed as the whole optical transmission line is designed so as to yield an absolute value of 0.02 ps/nm2/km or less. This is because of the fact that it can make transmission characteristics more uniform over the whole wavelength band in use at the time of WDM transmission. Preferably, at the predetermined wavelength, the average polarization mode dispersion observed as the whole optical transmission line is set to 0.2 psxc2x7kmxe2x88x92xc2xd or less. This is because of the fact that it can restrain transmission characteristics from deteriorating due to the polarization mode dispersion.